This invention relates to the purification of phosphonitrilic chloride polymer to obtain substantially cyclic trimeric and tetrameric phosphonitrilic chloride polymers. The preparation of phosphonitrilic chloride polymers is well known in the art. However, certain preferred phosphonitrilic chloride polymers, particularly the cyclic trimer and tetramer, have recently become important for uses in high temperature functional fluids, flame retardant additives, polymers and as intermediates for such uses. Because the prior art processes for producing phosphonitrilic chloride polymers are not specific to these cyclic trimer and tetramer species, there is a need to separate the cyclic trimer and tetramer from mixtures with other higher cyclic and linear phosphonitrilic chloride polymers.
Purification processes known in the prior art include fractional distillation, crystallization and solvent extraction; for example, U.S. Pat. No. 2,788,286, Teja et al., teach the fractional distillation of cyclic trimer and cyclic heptamer from mixed phosphonitrilic chloride polymers at 12 millimeters of mercury and 135.degree.C after preparation of a phosphonitrilic chloride polymer mixture by reacting ammonium chloride and phosphorus pentachloride in tetrachloroethane and quinoline at 145.degree.C, evolving HCl and then distilling off the solvent. In another process for producing phosphonitrilic chloride mixtures, U.S. Pat. No. 3,367,750, to Jaszka et al., teaches producing phosphonitrilic chloride polymers by reacting ammonia and chlorine in an inert solvent to form finely divided ammonium chloride, introducing phosphorus pentachloride into the dispersion and reacting to form phosphonitrilic chloride polymer. In the Jaszka et al. U.S. Pat. No. 3,367,750 process, the purification is carried out by filtering the reaction product, distilling the filtrate to remove monochlorobenzene solvent and washing the crude phosphonitrilic chloride with petroleum ether to extract cyclic polymers and crystallizing the cyclic phosphonitrilic chloride from the petroleum ether. Further, Jaszka et al. in U.S. Pat. No. 3,372,005 teach purifying crude phosphonitrilic chloride polymers at less than 170.degree.C by passing an inert gas through molten crude phosphonitrilic chloride polymer to obtain a polymer laden inert gas. The phosphonitrilic chloride polymer laden inert gas is then contacted with solvent to separate the distilled phosphonitrilic chloride polymer from the inert gas and then the solvent is vaporized from the solution to produce trimeric and tetrameric cyclic phosphonitrilic chloride polymer crystals. In still another process, Jaszka et al. in U.S. Pat. No. 3,379,510 teach a method for separating cyclic phosphonitrilic chloride trimer and tetramer from a liquid mxture of phosphonitrilic chloride polymers and solvent by forming a moving agitated film on a wiped film evaporator wall, heating the film to vaporize the trimer, tetramer and solvent and separating the vapor and liquid phases containing higher molecular weight polymers. The vapor phase is then cooled and the cyclic trimeric and tetrameric phosphonitrilic chloride polymers are separated from the solvent. In a variation on the Jaszka et al. U.S. Pat. No. 3,372,005 patent, Maund et al. in U.S. Pat. No. 3,677,720 teach a process for selectively purifying mixtures of phosphonitrilic chloride polymers by heating a mixture of process solvent and the polymer, flashing off solvent which is superheated and then countercurrent contacting the superheated inert solvent vapor with polymer so as to selectively vaporize cyclic trimer. The solvent vapor phase laden with trimer and some tetramer is separated from the molten polymer residue, condensed and the trimer and tetramer recovered from the solvent solution. In another type of process, U.S. Pat. No. 3,694,171 to Dreifus teaches contacting a solution of phosphonitrilic chloride in monochlorobenzene solvent with aqueous caustic, separating the resultant aqueous phase from the polymer solution, and then separating the resultant solution of cyclic trimer and tetramer from the aqueous phase.
In each of the above identified prior art processes, material handling is difficult or process equipment requirements are large and there is a need for a simpler process which has the advantage of omitting steps of the prior art processes and simplifying equipment requirements, thus lowering capital investment and improving process economics. The process of the present invention has these advantages while, nevertheless, producing product of the quality of the prior art or better.